Embodiments of the present invention relate generally to cooling systems for submersible pump systems and, more particularly, to submersible pump systems comprising submersible multi-stage motors with independent cooling systems.
Submersible pumps are typically driven by submersible motors. Generally, they are operable in a variety of applications in which both the pump and the motor are completely submerged in a well. A submersible motor for a submersible pump system has a stator that drives rotation of a rotor, generating heat during operation. This generated heat must be removed from the motor to prevent damage to the motor winding and extend the operational life of the motor. Submersible motors may be filled with a motor cooling fluid that transfers the heat from inside of the motor to the well fluid outside of the motor. The motor may include a cooling system that re-circulates and cools the motor cooling fluid so as to maintain a desired, cool temperature of the motor cooling fluid and the motor.
Deep-well submersible (DWS) pumping systems (also referred to as electric submersible pumps (ESP)) are especially useful in extracting valuable resources such as oil, gas, and water from deep well geological formations. In one particular operation, geothermal resources, such as hot water, can be retrieved from significant depths using DWS pumps. In a conventional configuration, a centrifugal pump and the motor that powers the pump are axially aligned and oriented vertically in the well. Because DWS pumping systems are relatively inaccessible (often completely submerged at distances up to or even more than a mile beneath the earth's surface), they must be able to run for extended periods without requiring maintenance.
Conventional submersible motors are generally not capable of withstanding the high operating temperatures and pressures associated with the DWS environment. Some applications require the pump to operate in surrounding liquid temperatures of over 100° C. or more. For example, in situations involving geothermal wells, the water being extracted from the earth may be up to 160° C. or more.
Further, submersible pump systems often use larger motors which demand greater power. Such motors typically have multiple stators built in series, creating a multi-stage motor in order to reduce the well diameter. A common problem with conventional multi-stage motors is that they have only one cooling circuit to cool all of the stators. However, a single cooling circuit is generally not sufficient to maintain a cool temperature throughout a multi-stage motor. As a result, a heat gradient forms in the axial direction through the multi-stage motor. The temperature rises from one stator to the next, with the highest temperature at the top stator of the multi-stage motor. This can cause an alteration of the motor winding and can lead to early winding failure.
Thus, the effectiveness of the cooling system determines the maximum power that can be obtained from a motor of a particular size, as well as the lifetime of the motor, especially the motor winding. The better the cooling system, the more power can be obtained from the same motor size, and the longer the lifetime that can be expected. Conversely, the motor can be operated at the same power but with a lower inner temperature with an improved cooling system, which would significantly increase the lifetime and robustness of the submersible motor. Therefore, it would be beneficial for the lifetime of the motor to keep the heat gradient low in order to avoid hot-spots and damage to the winding insulation of the motor.
Thus, there exists a need for a cooling system that will maintain an acceptable temperature throughout a multi-stage motor.